The invention relates to a method for the production of three-dimensional microstructures.
In a method of the present type first a source material is applied on a substrate, with here the properties of the source material can be changed by exposure to electromagnetic radiation. In lithographic methods of prior art such source materials are provided as positive or negative resists, for example. Then, using locally resolved exposure of the source material a three-dimensional source structure is written, either by a sequential layer-for-layer method or in a single step. Depending on the source material used (for example positive or negative resist) the source structure is removed from the source material or the source material is removed except for a source structure. Subsequently the source structure is molded from a target material, which the microstructure to be produced is to be made from. Conventional target materials are particular metals, metal alloys, semiconductors, and ceramics.
Microstructures in the sense of the present invention are such structures that extend perpendicular in reference to a substrate and exhibit an extension ranging from more than approx. 0.1 μm to approximately 1 mm. In the sense of the present invention microstructures are called “three-dimensional” which in a top view of the substrate exhibit undercuts inwardly or outwardly, with here a spatial periodicity not being required.
In prior art microstructures are usually molded via a matrix produced from a positive resist. The exposure of positive resists leads to a chemical or physical change at the exposed site. This change may develop for example by the disintegration of chemical bonds. In order to allow the production of three-dimensional structures the light source is focused in the positive resist, namely such that the chemical bonds of the resist are only disintegrated in the focus. This can be achieved by a non-linear effect, i.e. either the resist reacts non-linear and shows a limit of light intensity, below which no exposure may occur, or the method of the two- or multi-photon polymerization is used, i.e. the probability for exposure in the focus is increased by the intensity increased here in reference to the environment. The disintegration of chemical bonds in the positive resist leads to a selective solubility thereof in the subsequent development step. Only the exposed areas are washed out.
A respective method is known for example from the publication Gansel, Thiel, Rill, Decker, Bade, Saile, von Freymann, Linden, Wegener: Gold Helix Photonic Metamatertial as Broadband Circular Polarizer, Science, Volume 325, 1513 (September 2009). Here, three-dimensional microstructures are produced from gold such that positive resists are exposed and developed by a direct laser writing method based on a multi-photon absorption mechanism. By the exposure and/or the writing of a source structure hollow spaces develop during the development at those locations of the positive resist to which the laser had been focused. Subsequently, an electrolyte gold is galvanically precipitated into these hollow spaces.
Based on the properties of the positive resist this method of prior art is however subject to limitations: positive resists are generally unable to form well-defined structures with a height of more than a few 10 μm, because the resist itself cannot be applied with a thickness of more than a few 10 μm without the quality of the source structure being considerably compromised. Although it can be multiplied by repeated applications, this however increases the complexity with reduced quality of the source structure. Further, positive resists show the characteristic of offering relatively low resolution for optic lithography processes (particularly in three-dimensional exposure), so that structures finer than approximately 500 nm cannot be produced with the quality required.
Although negative resists are not subject to the above-mentioned limitations of the positive resists, however here the entire volume of the resists must be exposed, except for the areas, which later shall be separated as the target material. The increase in exposure period involved here is acceptable at best at laboratory scales. Alternatively, several methods can be used simultaneously or sequentially in order to minimize the writing time, for example by template exposure of large resist volumes on the substrate, however in this case complexity increases.
Additional examples for methods to produce three-dimensional microstructures, which however are not directly equivalent to the method of the present type, are disclosed in the publication “LIGA and its applications”, in Advanced Micro & Nanosystems, Volume 7, 1st edition, Wiley-VCH, 2009, and in the publication “Stereolithographie—das bekannteste Verfahren des Rapid Prototyping (Stereolighography—the best-known method for rapid prototyping)”, Volume V146092, Grin Verlag, 2010. In the first-mentioned publications of prior art, two and three-dimensional structures are produced by exposing resists, galvanic precipitation, and micro-molding. For this purpose, photo-templates are used, which are placed onto the resists. This way, high-aspect 2D-structures can be produced; by a targeted exposure from various angles and repeated exposure even more complex 3D-structures can be realized. In the second publication mentioned regarding prior art a stereo-lithographic method is described by which microstructures can be produced as well. Here, in a bath filled with the basic monomers of a light-sensitive plastic, thin plastic layers are cured by a laser. After each curing step the work piece is lowered a few millimeters into the liquid and returned to a position located lower by the amount of a layer thickness. This way, a three-dimensional structure is generated layer by layer.